Historically, the distinction between adjectives and nouns is a fairly recent one—not entrenched before the 18th century. The classical grammars referred to nominals, which included adjectives and nouns.
In addition, Greek, unlike English but like many other languages, can routinely use adjectives on their own without a noun. In fact, neuter adjectives were how Classical philosophers referred to abstractions: τὸ ἴσον, “the equal”, was how someone like Plato would refer to Equality.
There is only one way in which Ancient Greek adjectives differ morphologically from nouns. Feminine 1st declension nouns that are accented on the penult in the singular are accented on the penult in the plural: κινάρα ‘artichoke’, κινάραι ‘artichokes’. If a feminine adjective is accented on the penult in the singular, it is accented on the antepenult in the plural, by analogy with the masculine plural: δεύτερος, δευτέρα: δεύτεροι, δεύτεραι (not: δευτέραι).
With typical cluelessness, 19th century grammars say that ethnic names are an exception to the accentuation rule for nouns: Ῥοδία “Rhodian woman”, Ῥόδιαι “Rhodian women”, not the expected Ῥοδίαι. But of course, that isn’t an exception at all. That just shows you that to Ancient Greeks, Ῥόδιαι was not a noun but an adjective.